Q.In soil, water available for absorption by root is ...................

Combined water (bound water) is the water that is chemically or physically bound tightly to soil particles, colloids or cell components (such as by adsorption, hydration, or as water of crystallization) and is not fre...

Combined water is tightly bound water (in soil/cells) that plants cannot readily absorb, unlike freely available capillary water.

Soil water exists in different forms with differing availability to plants. Combined (bound) water refers to water molecules that are held very tightly — either chemically combined with soil mineral particles (as water of hydration/crystallization) or physically adsorbed as a thin film on the surface of soil colloids/clay particles by strong adhesive forces — such that it cannot move freely and is essentially unavailable for absorption by plant roots under normal conditions. This is in contrast to capillary water (held in soil pores by capillary/surface-tension forces, and the main source available to plants) and gravitational water (which drains away freely under gravity).

Imbibition — soil water first enters the porous cell wall of the root hair by imbibition, after which it moves further into the cytoplasm and vacuole by osmosis.

The very first step of root water absorption is imbibition by the root hair cell wall.

Root hairs are the primary absorbing units of the root system. Water in the soil first wets and is imbibed by the hydrophilic cellulose/pectin cell wall of the root hair (imbibition), which lowers the wall's water potential and draws water in physically, even before any living membrane process begins. Once inside the cell wall, water crosses the plasma membrane into the cytoplasm and central vacuole mainly by osmosis, moving down a water-potential gradient from the soil solution (higher ψ) toward the more negative ψ of the root hair cell sap, and then continues cell-to-cell (apoplast/symplast) toward the xylem.